I think a thumb on a backhoe would be good if the person doesn't want to buy or rent an excavator. The only reason my friend has a thumb on his hoe is because he is older than a mammoth and does everything with his hoe. Because of that he is not very skilled at an excavator. I personally wouldn't put one on a hoe.
I have an intrensic issue with renting... and given that I have a backhoe and its reach of 18' is enough to do the work needed I use it instead of renting. I know I am not alone with this one because this feeling came from my father.
Also for the cost of running my backhoe instead of renting an ex, I have aditional money to play with.
This are just my feelings... :beatsme